Remote wireless sensor nodes for the Internet of Things (IoT) rely on duty-cycling to achieve extremely low average power consumption. However, these approaches generally require an accurate wakeup timer. Such a timer should avoid off-chip components, such as quartz crystals and occupy minimal area so as to save costs and reduce module size. The wakeup timer should also consume ultra-low power (<1 μW), since it is continuously active, while operating at a low supply voltage for compatibility with a wide range of energy sources (e.g., button batteries, energy scavengers, etc.) and to simplify power management. Because of size and power limitations, RC oscillators are a conventional choice. However, the frequency stability of RC relaxation oscillators is limited by the delay of power-hungry continuous-time comparators, which are vulnerable to process, voltage, and temperature (PVT) variations. Oscillators based on frequency-locked loops (FLL) circumvent such limitations, but they generally rely heavily on analog-intensive circuits (e.g., OP-AMP), which require significant power, area and a high supply voltage. Hence, such approaches are not amenable to technology scaling in terms of area and required supply voltage.